Unit ( Metric) Problem : Not able to get full 20cm
If I understand it correctly here we go:

First issue.

Blender doesn't have so precise modeling system as CAD software. So if you want correct edge lenght you will need to do this manually.

While scaling object in Edit Mode with selected edge lenght you can hold Shift to get more precision. Zooming in and holding cursor (before hitting S) away from object center could help a bit.



Second.

In Edit Mode change Pivot Center to Individual Origins then in tools choose Extrude Individual, Right Click and S to scale down each face.